Circuit analyzing system

ABSTRACT

A circuit analyzing system includes a current transformer for extracting current from a phase or neutral conductor of a three phase line, a filter connected to the output of the current transformer and functioning to substantially trap current at the fundamental frequency while substantially passing currents at various harmonic frequencies thereof, and signal processing circuitry connected to the output of the filter to perform various functions with the harmonic frequency current input thereto. Among these functions, as shown in various preferred embodiments, are measurement of the harmonic signal in RMS and conveyance of the output to a display instrument for a user or utility&#39;s convenience in monitoring harmonic frequency circuits, which have become more corrosion with the advent of electronic equipment having internal power supplies such as personal computers, work stations, printers, and the like. Other functions include a user or utility&#39;s connecting a demand circuit mechanism to the measured currents (preferably RMS/harmonic frequency currents), and/or using comparator circuitry to sense the measured harmonic frequency currents and, if they exceed a desired level set by the user or utility, to output a signal to a tripping device such as a power contactor or relay which will in turn institute load shedding.

This is a continuation of application Ser. No. 08/373,684 filed on Jan.17, 1995 and now U.S. Pat. No. 5,499,155 issued Mar. 12, 1996, which wasa continuation of application Ser. No. 08/001,885 filed Jan. 8, 1993 andnow U.S. Pat. No. 5,383,084 issued Jan. 17, 1995.

BACKGROUND OF THE INVENTION

The invention relates to a system which will detect and measure theharmonic content of 60 Hz current.

A problem which is increasing in importance is the amount of harmoniccurrent generated in branch circuits. Such harmonic current is producedby most electronic power supplies. With the ever increasing number ofpersonal computers, work stations, printers, etc. connected to a userfacility's power circuit, the level of harmonic current can overloadbranch circuit wiring. Also, excessive harmonic current is detrimentalto efficient utility power transmission and distribution.

To more precisely describe the problem caused by harmonic current, powersupplies for use in computers and other electronic equipment sometimesutilize power line current in a non-sinusoidal form. The result is thatharmonic currents are taken from the power line. This phenomenon can beparticularly bothersome with regard to the current in the neutral lineof a 3 phase circuit. In such a circuit, the harmonics do not canceleach other out as do fundamental sine currents. The neutral line, if notdesigned to compensate for this harmonic current, may overheat. Also,the utility supplying the user may find that its station and substationequipment, as well as its neutral wire, (transmission and/ordistribution) is undersized in view of the harmonic current.

This phenomenon is widespread because most buildings were designedbefore the proliferation of electronic equipment with power suppliessuch as hereinbefore described, and consequently they are vulnerable tocircuit failure within the building.

To more fully describe the problem presented by currents and harmonicfrequencies, and assuming a fundamental frequency of 60 Hz, theharmonics would be 120, 180, 240, 300, etc. Hz. In general, the problemis caused mainly by the "harmonics that are multiples of 3," i.e., 180,360, 540, 720, etc. Hz. These harmonics do not cancel in the neutralconductor. Thus, if the wiring was originally "sized" for pure sinewaves, as was the case before the proliferation of electric powersupplies, the neutral conductor may be undersized for modern loads, suchas p.c.s, terminals, workstations, printers, etc. Additionally, aspreviously implied, the utility generating and distributing equipmentcan be overheated by the harmonic content of the load currents.

A similar problem exists with regard to the utility in that it wouldlike to be able to measure and detect harmonic current and thus produceor buy equipment to take care of the problem. The benefit to the utilitywould lie in its ability to charge customers for remedying the problemscaused by this harmonic current.

SUMMARY OF THE INVENTION

A primary object of the invention is to provide an economical means formeasuring harmonic frequency currents within branch circuits of a systemand, if desirable, either disconnecting the circuits by means of loadshedding and/or having the supplying electric utility charge a premiumfor excessive demand caused by harmonic frequency currents in a user'sbranch circuits.

This and other objects of the present invention are achieved by thesystem of the present invention which comprises a filter circuitconnected to the neutral conductor of a 3 phrase circuit having loads ofthe type hereinbefore described, wherein the filter circuit connected tothe neutral conductor is basically a 60 Hz trap which substantiallyeliminates 60 cycle current and also functions to substantially passhigher harmonic frequency currents, i.e., it rejects or reduces currentsby 60 decibels(db.) at 60 Hz and less than one db. at 180 Hz and thehigher harmonic frequencies, i.e. 240 Hz, 300 Hz, and 360 Hz, etc.

More specifically, the filter circuit of the present invention comprisesthe aforementioned 60 Hz trap filter whose input is connected to acurrent transformer which measures neutral current and conveys it to the60 Hz trap filter which extracts energy at all frequencies above thefundamental frequency of 60 Hz as previously described. These extractedharmonic currents are summed in a signal processor of some type(preferably in a root mean square or RMS type device). The informationthus obtained is then available for a display to indicate RMS neutralconductor current. Also, the information (harmonic RMS current) can besummed over a fixed time by a demand meter such that a utility can usethe demand meter information to collect additional revenue fromcustomers with excessive harmonic eneration. Also, the output availablefrom the signal processing circuitry can be used to shed loads forparticularly high harmonic current situations.

On the other hand, the same type of circuit could be used by a powerconsumer in order to determine which equipment and/or branch circuitgenerates the most harmonic current. Such information could then be usedby the consumer to install filters such as inductors or other electricmeans to reduce the harmonic content of the current received from theutility.

In a preferred embodiment, the filter circuit could comprise a Twin-Tfilter, although other preferred embodiments contemplate the use ofother types of filters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic of the circuit analyzing system of thepresent invention.

FIG. 2 is a schematic of a preferred embodiment of the filter circuit ofthe circuit analyzing system of FIG. 1.

FIG. 3 is a schematic of another preferred embodiment of the filtercircuit of FIG. 1.

FIG. 4 is a schematic of another preferred embodiment of the circuitanalyzing system of the present invention.

FIG. 5 is a schematic of yet another embodiment of the circuit analyzingsystem of the present invention.

FIG. 6 is a schematic of still another embodiment of the circuitanalyzing system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a general schematic of the circuit analyzing system of thepresent invention. The system is generally identified by referencenumber 10. As shown therein, input phase lines 12, 14 and 16, as well asneutral line 18, are connected to loads 20. Connected to neutral line 18is transformer 22 which measures neutral current. The output therefromis conveyed to filter 24 which extracts energy at all frequencies abovethe fundamental frequency (60 Hz in the U.S.). The odd harmonicfrequencies (i.e., 180 HZ, 300 HZ, etc.) thus extracted are summed,preferably in a root mean square (RMS) scaled mechanism, which formspart of the signal processing block 26 shown in FIG. 1. This informationis then available to be conveyed to display apparatus 28 which canindicate the RMS neutral line current. These harmonic currentfrequencies can be summed within the signal processing module 26 by ademand mechanism. In this particular instance, since the demand circuitmechanism is attuned to summing harmonic current, it can be used by theutility to collect revenue from customers with excessive harmoniccurrent generation.

In addition to the use of the circuit system of FIG. 1 by a utility,such a circuit can be used by a power consumer in order to determinewhich equipment and/or branch circuit is generating the most harmoniccurrent. This information could then be used by the consumer to installfilters such as inductors or other electronic means to reduce theharmonic content of the current taken from the utility.

Yet another application of the circuit analyzing system of FIG. 1 is toconvey the summed harmonic current to load shedding elements such aspower contactor 30 in the event that the harmonic current generated isso large as to endanger the power continuity to other parts of thesystem. As shown in FIG. 5, power contactor 30' has contacts such ascontacts 92, 94 and 96 which contacts can be used to respectively openlines 12, 14, and 16 in the event of harmonic current overload upon asignal from power contactor control on/off 102.

FIG. 2 shows an embodiment of the circuit analyzing circuit of FIG. 1using a Twin-T circuit. This Twin-T circuit, generally designated withreference numeral 38, comprises one T network including capacitors 40and 42 as well as resistor 44. A second T network connected in paralleltherewith comprises resistors 46 and 48 as well as capacitor 50. Thenetwork functions such that, if the reactance and resistances areproperly chosen, these networks may be used to pass through or reject aparticular frequency. In the present application of course they are usedto substantially rejeot the fundamental frequency (60 Hz) and tosubstantially pass therethrough the aforementioned harmonic frequencies.

The advantages for using a Twin-t filter are those of component cost andsize. At 60 Hz, inductors such as would be used in a conventional LCfilter are large and expensive. The Twin-T circuit uses only resistorsand capacitors. The basic resonant equation uses the product of "RC" andtherefore large values of "R" can be used with small values of "C" togive a low frequency.

FIG. 3 shows a preferred embodiment of the invention wherein a sharper(narrower) frequency response curve is achieved by combining operationalamplifiers with a Twin-T circuit. As shown therein, the circuitcomprises a "Single-T" arrangement showing resistors 52 and 54, togetherwith capacitor 56. This circuit is arranged in parallel with the other"T" arrangement comprising capacitors 58 and 60 together with resistor62. The harmonic signal is then conveyed from the Twin-T network throughresistor 64 to operational amplifier 66 which has its positive inputterminal connected through resistor 68 to ground and has positive andnegative voltage outputs 70 and 72, and has resistor 74 connected inparallel therewith. The output of operational amplifier 66 is connectedthrough resistor 76 to the negative terminal of operational amplifier 78which has its positive terminal connected to ground through resistor 79and has positive and negative voltage outputs 80 and 82, and hasresistor 84 connected in parallel therewith. The output of theoperational amplifier 78 is connected to the Twin-T network throughresistor 62 and capacitor 56 to feedback a signal thereto. As statedpreviously, a sharper frequency response i.e., a more "narrow" frequencycurve is achieved by this combination of operational amplifiers with theTwin-T network.

FIG. 4 shows one preferred embodiment of the circuit analyzing systemshown generally in FIG. 1. In this embodiment current transformer 22 isconnected to neutral line 18 and conveys current to 60 Hz trap filter 24which substantially blocks 60 Hz frequency signals but substantiallypasses all harmonic signals. These signals are conveyed to a true RMSdetector 86 which has a DC output connected to a digital voltmeter anddisplay 88. Consequently, a user or a supplying utility can obtain adirect readout of the harmonic current in a specific neutral line. Itshould be noted that the current in any other line can be analyzed bythreading the wire carrying the current through current transformer 22.Also, other means can be used for detecting the current than the currenttransformer, and the current transformer 22 does not have to be a toroidbut can be another transformer configuation.

FIG. 5 shows another preferred embodiment of the circuit analyzingsystem of the present invention. In this embodiment the harmonic currentto separate loads 89 and 90 can be monitored by using separate phaseconnections to respective loads 89 and 90 and using switches 92, 94 and96 to facilitate the desired connection between respective phase lines12, 14 and 16 to respective loads 89 and 90. This circuit includes, asdoes the circuit of the previous preferred embodiments, a currenttransformer 22 connected to neutral line 18 and 60 Hz trap filter 24which substantially blocks 60 Hz transmission and outputs only currentat substantially harmonic frequencies. These harmonic frequencies areconveyed through RMS detector 86 which has a DC output connected to acomparator 98. The comparator 98 has a voltage reference input 100 whichis preset at a level to shed load 89 when the harmonic current isexcessive. This load shedding is accomplished by power contactor 30'consisting of contacts 92, 94, and 96 which opens these contacts inresponse to the signal from power contactor control 102, thus cuttingoff current to load 89 when the harmonic content is excessive. Thisphenomenon occurs when the output of the true RMS detector 86 exceedsthe voltage reference input from module 100, which voltage reference canbe set by the utility or the user. Then the power contactor control 102goes into a control "off" mode and causes contactor 30' to open thecontacts 92, 94 and 96 as hereinbefore explained.

The preferred embodiment of FIG. 6 contemplates use by a electricutility to determine excessive harmonic content in the signals it issupplying over relatively short periods of time during a utility billingcycle. In this embodiment, as in the previous embodiments, a currenttransformer 22 senses current in the neutral line 18 and conveys it to a60 Hz trap filter 24 which removes the 60 Hz signal and extracts theharmonics thereof. The harmonic components of the current are conveyedto a true RMS detector 86 which in turn conveys current to a demandmeter 122. The demand meter 122, which is calibrated in ampere-hoursdetermines harmonic current for a preset period of time, for example 1hour. At the end of the testing period, the total ampere-hours arecompared with the ampere-hours from the previous hour. The hiSher ofthese two values is then stored to be compared to the ampere-hours ofthe next hour. The ampere-hour measuring instrument is then reset tozero and a new cycle begins. In this manner, at the end of the billingperiod, the utility knows the maximum usage of harmonic currents and canbill accordingly.

It will be appreciated that variations and alterations to the disclosedpreferred embodiments of the invention can be made without parting fromthe spirit and scope of the invention.

We claim:
 1. A circuit analyzing system comprising:a) at least one loadto be supplied with current from a three phase AC power supply; b) threephase conductors and one neutral conductor connecting an AC power supplyto said at least one load; c) means in said three phase conductors toopen said three phase conductors to said at least one load in responseto an actuating signal; d) means for extracting current from saidneutral conductor; e) a high pass Twin-T filter coupled to said meansfor extracting current to block the passage therethrough of current atthe fundamental frequency of said AC power supply and pass harmonicfrequency currents which are harmonics of the fundamental frequency; f)said high pass Twin-T filter having an input coupled to said means forextracting current, an output at which said harmonic frequency currentsare provided, and a feedback input; g) a first operational amplifierhaving first negative and first positive input terminals, a first outputterminal and a first feedback resistor coupled between said first outputterminal and said first negative input terminal, said first negativeinput terminal further coupled to said Twin-T filter output through aresistor; h) a second operational amplifier having a second negative anda second positive input terminal, a second output terminal and a secondfeedback resistor coupled between said second output terminal and saidsecond negative input terminal, said second negative input terminal alsocoupled to said first output terminal of said first operationalamplifier through a coupling resistor and said second output terminalcoupled to said feedback input of said Twin-T filter; i) a signalprocessor coupled to said output of said Twin-T filter to add all theharmonic frequency currents together and provide an actuating signal tosaid means to open said three phase conductors when the total of saidharmonic frequency currents exceeds a preset value.
 2. A circuitanalyzing system, as defined in claim 1, wherein said means forextracting current from said neutral conductor is a toroidal transformerand said neutral conductor passes through said toroidal transformercomprising a one turn input winding and said toroidal transformer has anoutput winding coupled to said input of said Twin-T filter.
 3. A circuitanalyzing system, as defined in claim 1, wherein said signal processorfurther comprises:a) means for generating a selectable variable voltagereference signal; and b) a comparator to receive the added harmonicfrequency currents and a selected variable voltage reference signal andproduce an actuating signal when the added harmonic frequency currentsexceed the selected variable voltage reference signal.
 4. A circuitanalyzing system, as defined in claim 2, wherein said Twin-T filtercomprises a first T filter section comprising a first resistor and asecond resistor in series and a first capacitor connected at one end tothe junction between said first resistor and second resistor and asecond T filter section connected in parallel with said first T filtersection and comprising second and third capacitors connected in seriesand a third resistor connected at one end to the junction between saidsecond and third capacitors, said output winding of said toroidaltransformer coupled to the other end of said first capacitor and thirdresistor and to said first resistor and said second capacitor.